FIG. 3 shows a prior art semiconductor laser device which is disclosed in J. Appl. Phys. 60(7), 2633(1986).
In FIG. 3, an n type Al.sub.x Ga.sub.1-x As (0.ltoreq.x.ltoreq.0.60) lower cladding layer 2 having carrier concentration of about 10.sup.17 cm.sup.-3 is disposed on an n type GaAs substrate 1. A p type or n type Al.sub.y Ga.sub.1-y As (0.ltoreq.y.ltoreq.0.45 and y&lt;x) active layer 3 having a carrier concentration of about 10.sup.17 cm.sup.-3 is disposed on the n type Al.sub.x Ga.sub.1-x As lower cladding layer 2. A p type Al.sub.x Ga.sub.1-x As upper cladding layer 4 having a carrier concentration of about 10.sup.17 to 10.sup.18 cm.sup.-3 is disposed on the p type or n type Al.sub.y Ga.sub.1-y As active layer 3. An n type GaAs blocking layer 6 is disposed on the p type Al.sub.x Ga.sub.1-x As upper cladding layer 4. A p type GaAs contact layer 7 is disposed on a stripe ridge portion 9 of the p type Al.sub.x Ga.sub.1-x As upper cladding layer 4. Reference numerals 10 and 11 designate an n side and p side electrode, respectively.
The production process of the device will be described.
First, an n type Al.sub.x Ga.sub.1-x As lower cladding layer 2, an active layer 3, a p type upper cladding layer 4, and a contact layer 7 are epitaxially grown on an n type GaAs substrate 1. Thereafter, the upper cladding layer 4 and the contact layer 7 are wet etched using for example a mixed solution of hydrogen peroxide, ammonia, and water, thereby producing a stripe ridge comprising the upper cladding layer 4 and the contact layer 7. The inclination of the exposed side surface of the stripe ridge can be controlled to a desired value by selecting a proper etching solution. In this case the angle is desirably 54.5.degree.. Thereafter, n type current blocking layers 6 are grown on the exposed surfaces of the upper cladding layer 4. Thereafter, an n side electrode 10 and a p side electrode 11 are produced on the n type GaAs substrate 1 and on the exposed surface of the p type contact layer 7 and the n type blocking layers 6, respectively.
The device will operate as follows.
When a negative and positive voltages are respectively applied to the n side and p side electrodes 10 and 11, a current flows predomenantly through the ridge portion 9. Then, in the active layer 3 the neighborhood of the ridge portion 9, electrons and holes are respectively injected into the active layer 3 and light emission occurs due to the recombination of electrons and holes. When the injection current is increased above a threshold current, stimulated emission starts and laser oscillation occurs.
In the prior art semiconductor laser of such a construction, however, vertical transverse mode oscillation, i.e., the mode perpendicular to the interface between the substrate and the lower cladding layer, is controlled by the refractive index waveguide structure created by the differences in refractive indices of layers 2, 3, and 4. On the other hand, horizontal transverse mode oscillation, i.e., the mode parallel to the interface between the substrate and the lower cladding layer, is controlled by the effective refractive index difference, that is the, so-called rib waveguide. Although the device of FIG. 3 is of a refractive index type, there is no built-in refractive index difference in the horizontal direction.
Accordingly, this device has a higher astigmatism than a device having built-in refractive index differences in both the vertical and horizontal directions, although it has a less astigmatism than the gain waveguide laser. Herein, the refractive index waveguide laser generally has less astigmatism than the gain waveguide laser because the light emitted from the refractive index waveguide laser begins to be broaden where the light is entirely reflected inside the crystal and the light is emitted having parallel wave fronts.